1. Technical Field
This invention relates to the technology of cleansing the emissions of an automotive internal combustion engine by effectively removing not only nitric oxide in the emissions but also hydrocarbons and CO when the engine exhaust is lean (excess of oxygen).
2. Discussion of the Prior Art
Three-way catalysts (TWC) have been commercially employed for some time in the cleansing of emissions from fossil-fueled automotive engines that are operated at or slightly rich of stoichiometry. These catalysts have usually been employed as a combination of noble metals including platinum, palladium, and rhodium. If the same three-way catalysts were to be employed to cleanse emissions from a lean-burn engine, the excess oxygen in the exhaust of such an engine would render the operation of a conventional TWC ineffective, particularly with respect to nitric oxides.
The prior art has developed several oxidation catalysts for removal of CO and hydrocarbons which will function in an excess oxygen environment; examples of such oxidation catalysts include noble metals such as platinum and palladium, and base metals such as copper, cobalt, and chromium. Unfortunately, such oxidation catalysts are not effective in reducing nitric oxide or other reducible elements of the emissions.
The prior art has discovered certain selective catalytic reduction agents such as ammonia and urea, which are highly effective in reducing nitric oxide from stationary source emissions, particularly when deployed over titania supported catalysts such as V.sub.2 O.sub.5 -TiO.sub.2. Unfortunately, such agents are unsuitable for vehicular use since they are toxic and restrict the conditions for use.
Recently, transition metal ion exchanged zeolites have been deployed as catalysts for vehicular use, particularly in diesel engines (see German patents 3,642,018 and 3,735,151). Such zeolite catalysts suffer from high temperature degradation and poor NO conversion efficiency at low temperatures; moreover, such catalysts require that the engine be run rich at least temporarily to generate sufficient reductants in the emission flow.
Even more recently, tests have been made using propane as a reductant over an alumina catalyst to reduce NO in an oxygen-rich flow (see "Selective Reduction of Nitrogen Oxides With Hydrocarbons Over Solid Acid Catalysts In Oxygen-Rich Atmospheres", Y. Kintaichi, H. Hamada, M. Tabata, M. Sasaki, T. Ito, Catalysis Letters, Vol. 6, 1990, pp. 239-244). Conversion of NO was low even under the ideal conditions used (323 ppm propane, low space velocity of 3000 hr.sup.-1, extremely low R, and an absence of contaminants in the emission flow). Presence of emission contaminants can reduce NO conversion efficiency in half, making the results even more disappointing. Moreover, each of the above references fails to disclose a catalyst system that is suitable for vehicular on-board use and attain desirably high conversion efficiency of NO at high space velocities and lean-burn conditions.